1. Field of the Invention
The invention relates to a multiband antenna and, in particular, to a multiband antenna capable of being operated in broadband range.
2. Description of Related Art
Wireless communication systems have a lot of progress in recent years, presenting great potential and business opportunity. Their techniques and bands are not completely the same. Each of these systems plays an important role in a distinct area and market. However, this phenomenon causes troubles and inconvenience to both system suppliers and consumers. One disadvantage is that different communication systems use different frequencies, such as GSM900, PCS1900, and Universal Mobile Telecommunications System (UMTS).
For the convenience of users, manufacturers have devoted a lot of manpower to develop products integrated with multiple band functions. However, the first difficulty that has to be overcome is the antenna. The antenna can be regarded as the beginning and end of wireless communications. Its performance directly affects the communication quality. As modern electronic devices are light and compact, the antennas also become smaller and hidden inside mobile communication devices. Since the planar inverted-F antenna (PIFA) has a length of ¼ wavelength, the sizes of antennas can be greatly reduced. Therefore, it is widely used in the design of built-in small antennas.
The PIFA that works in a single frequency can be found in, for example, U.S. Pat. No. 5,764,190. To enable multiband usage of the PIFA, the radiation metal sheet is cut with a V-shaped notch or U-shaped notch.
Another multiband antenna is shown in FIG. 1. The antenna includes a first radiation part A, a second radiation part B, and a ground part C. The first radiation part A and the second radiation part B are extended from two opposite side edges of the same end of the ground part C. The first radiation part A includes a first conducting sheet A1 parallel to the ground part C and a first connecting part A2 that is connected between the first conducting sheet A1 and the ground part C. The second radiation part B includes a second conducting sheet B1 parallel to the ground part C and a second connecting part B2 that is connected between the second conducting sheet B1 and the ground part C. The first conducting sheet A1 and the second conducting sheet B1 are extended from the first connecting part A2 and the second connecting part B2, respectively, toward the same direction.
Although the above-mentioned antenna can achieve the multiband operations, it has the following disadvantages. The distance between the first conducting sheet A1 and the second conducting sheet B2 is too close. Therefore, the bandwidths in low and high frequencies are insufficient. The antenna thus cannot effectively cover multiple system bands. During the real production process, the small distance also results in large errors and a lower yield. Moreover, as a conventional PIFA, a feed cable and a feed point on the antenna are close to the first connecting part A2. There is an upper limit in the antenna bandwidth, unable to achieve the broadband effect.
To solve the above-mentioned problems, the invention provides a novel means for a multiband antenna with the broadband function. The invention uses a radiator as the primary antenna radiation structure. The radiator has several sections of conductors and connecting conductors, thereby producing multiple resonant modes and multiband operations. Through coupling, electrical signals are fed into the antenna radiator to improve the bandwidth restriction of the conventional PIFA. At the same time, using two extension conductors, the surface current distribution and impedance variation of the antenna can be effectively controlled, so that the antenna has both the broadband feature and high radiation efficiency. In addition to the novel structure, the antenna also achieves the multiband operations, greatly enhancing the bandwidth and efficiency thereof. The disclosed antenna is thus compatible with multiple system bands and has a lot of industrial values.